1. Field of the Invention
The invention relates in general to magnetic resonance imaging (MRI), and in particular to devices for in vivo MRI.
2. Related Art
Minimally invasive surgical techniques often involve introducing a medical device e.g. an endoscope in any body lumen (natural or man-made) to provide an optical view of anatomy of interest. Surgical tools such as biopsy needles, incision/suturing devices, etc are used under optical guidance of the endoscope. The limitation of this technique is that the field of view (FOV) is limited in front of the device, in some cases by the end of the cavity. In particular, nothing can be seen beyond the surface of the tissue surrounding the endoscope. This poses a limitation for the operating surgeon, limiting the efficacy of the procedure. One approach to circumvent this problem is to employ imaging systems relying on signals other than visible light to generate an image of surrounding tissue. One such system is magnetic resonance imaging (MRI).
MRI is a well known, highly useful technique for imaging matter. It has particular use with imaging the human body or other biological tissue without invasive procedures or exposure to the harmful radiation or chemicals present with x-rays or CT scans. MRI uses changes in the angular momentum or xe2x80x9cspinxe2x80x9d of atomic nuclei of certain elements to show locations of those elements within matter. In an MRI procedure, a subject is usually inserted into an imaging machine that contains a large static magnetic field generally on the order of 0.2 to 4 Tesla although machines with higher and lower strength fields are being developed and used. This static magnetic field tends to cause the vector of the magnetization of the atomic nuclei placed therein to align with the magnetic field. The subject is then exposed to pulses of radio frequency (RF) energy in the form of a second, oscillating, RF magnetic field having a particular frequency referred to in the art as a resonant or Larmor frequency. This frequency is equal to the rate that the spins rotate or precess.
This second field is generally oriented so that its magnetic field is oriented in the transverse plane to that of the static magnetic field and is generally significantly smaller. The second field pulls the net magnetism of the atomic nuclei off the axis of the original magnetic field. As the second magnetic field pulses, it pulls the spins off axis. When it is turned off, the spins xe2x80x9crelaxxe2x80x9d back to their position relative to the initial magnetic field. The rate at which the spins relax is dependent on the molecular level environment. During the relaxation step, the precessing magnetization at the Larmor frequency induces a signal voltage that can be detected by antennas tuned to that frequency. The magnetic resonance signal persists for the time it takes for the spins to relax. Since different tissues have different molecular level environments, the differences in relaxation times provides a mechanism for tissue contrast in MRI. The magnetic resonance signal is detected in the form of a voltage that the precessing magnetization induces in an antenna placed nearby.
In order to image the magnetic resonance signal it is necessary to encode the locations of the resonant spins. This is performed by applying pulses of gradient magnetic fields to the main magnetic field in each of the three dimensions. By creating these fields, the location of resonant nuclei can be determined because the nuclei will resonate at a different Larmor frequencies since the magnetic field they experience differs from their neighbors. The magnetic resonance (MR) image is a representation of the magnetic resonance signal on a display in two or three dimensions. This display usually comprises slices taken on an axis of interest in the subject, or slices in any dimension or combination of dimensions, three-dimensional renderings including computer generated three-dimensional xe2x80x9cblow-upsxe2x80x9d of two-dimensional slices, or any combination of the previous, but can comprise any display known to the art.
MR signals are very weak and therefore the antenna""s ability to detect them depends on both its size and its proximity to the source of those signals. In order to improve the signal of an MRI, the antenna may be placed near or inside the subject to be imaged. Such improvements can enable valuable increases in resolution sensitivity and reduction of scan time. It may be desirable to have evidence of the MRI antenna itself on the MRI image to allow the individual inserting the MRI antenna to direct where it is going and to maneuver it with aid from the MR image. Such a benefit could be useful in medical procedures where MRI is used simultaneously to track the position of an intraluminal device and to evaluate the structures surrounding the lumen. For example, an intravascular catheter could be directed through a vessel using MRI to reach a targeted area of the vessel, and the MRI apparatus could further be used to delineate the intravascular anatomy or nearby tissue to determine whether a particular therapeutic intervention would be required. Using MRI to guide the catheter and using MRI further to map out the relevant anatomy could complement conventional angiographic imaging technology within an interventional radiology or cardiology or minimally invasive imaging suite. Once the catheter is directed to the desired anatomic target under MR guidance, and once the topography or other relevant anatomy of the target lesion is depicted using MRI, the clinician can make decisions about what type of intervention would be indicated, if any, and where the intervention should be delivered.
Many conventional vascular interventional procedures use X-ray imaging technology in which guidewires and catheters are inserted into a vein or artery and navigated to specific locations in the heart for diagnostic and therapeutic procedures. Conventional X-ray guided vascular interventions, however, suffer from a number of limitations, including: (1) limited anatomical visualization of the body and blood vessels during the examination, (2) limited ability to obtain a cross-sectional view of the target vessel, (3) inability to characterize important pathologic features of atherosclerotic plaques, (4) limited ability to obtain functional information on the state of the related organ, and (5) exposure of the subject to potentially damaging x-ray radiation.
MRI techniques offer the potential to overcome these deficiencies. However, many conventional intraluminal tools are not suitable for use in MRI machines since they contain steel or magnetic materials that can cause significant image artifacts in an MRI machine and can cause injury to a patient from unintended motion due to effects of the magnetic fields or induced Ohmic heating. Additionally, intraluminal devices made of non-magnetic materials (e.g., polymers) cannot easily be visualized by MRI. Even those antennae which have been fabricated for use inside a human body are not useful for many types of interventional procedures. Many of these devices are simply too large to be sufficiently miniaturized to allow the placement of an interventional device simultaneously with the antenna in a small vessel without causing injury to the subject. Furthermore, many of these devices are not useful because the antenna cannot work in conjunction with the range of interventional tools that are widely used in many types of procedures due to space and design considerations of the antenna. Such devices include, but are not limited to, such tools as balloon catheters for dilatation angioplasties, for stent placements, for drug infusions, and for local vessel therapies such as gene therapies; atherotomes and other devices for plaque resection and debulking; stent placement catheters; drug delivery catheters; intraluminal resecting tools; electrophysiologic mapping instruments; lasers and radio frequency and other ablative instruments. Conventional antennas fail in this regard because they have no method for allowing the loading and use of these devices concurrent with image acquisition by the antenna.
Various imaging coils for interventional MRI are known in the art. U.S. Pat. No. 5,738,632 to Karasawa, discloses an endoscope/rigidoscope with MRI coils located in the distal section of the device. U.S. Pat. No. 5,699,801 to Atalar et al (hereafter xe2x80x9cAtalar ""801xe2x80x9d) describes a loop antenna for interventional MRI and spectroscopy applications. The distance between the two sides of the loop is fixed and is approximately 2-3 mm. This separation is relatively small, which results in a received signal having a lower signal-to-noise ratio (SNR) than could be achieved with a larger separation. The caliber of such a device is limited, however, by the size of the smallest bodily structure through which it might be advanced. For example, if device according to Atalar ""801 were to be advanced through a vein with a diameter of 5 mm into a second vein with a diameter of 15 mm and finally into a heart chamber with a diameter to 40 mm, the device, its coil, and any other parts must all be less than 5 mm in caliber. If a device with a caliber of, for example, 25 mm were practiced according to Atalar ""801, it could not be used in the preceding example because its size is fixed, and it could not fit through the smallest structure in the desired path of the device.
In applications of such MRI coils, it would be desirable to introduce adjacent to the MRI antenna other devices including PTCA catheters, endoscopes, trocars, other minimally invasive surgical equipment or MRI antennae for the purpose of diagnosis or therapeutic intervention. The prior art does not provide for such a capability.
Also in applications of such MRI coils, it is desirable to introduce the MRI antenna into a cavity, access to which is available only through very narrow lumens. For example, access to chambers of the heart is limited by the caliber of blood vessels entering and exiting the heart. Thus, a low profile device is needed to gain access to such cavities. This necessity introduces all the limitations of existing low profile devices, primarily diminished SNR. In addition, if the narrow-lumen access pathway is a vascular structure, a device completely occluding that lumen might not be usable in that lumen since tissues whose blood supply depends on the patency of that vessel would be starved of oxygen. The prior art does not provide a means for an MRI antenna to make use of additional available space once the antenna has been fully advanced into a cavity with a lumen larger than its access structures, or for positioning an MRI antenna in a structure while leaving that structure at least partly patent throughout its length.
Catheters have long been used in the art as sleeves through which other medical devices may be advanced to an anatomical point of interest for examination, diagnosis, and intervention. However, advancement of the catheter requires constant monitoring to ensure that the catheter is being advanced through the correct structures, without kinking, causing injury, failing mechanically, and for other reasons known to one skilled in the art. Methods existing in the art for such monitoring include X-ray visualization of the catheter, and MRI tracing of a component of the catheter designed to be visible to an MRI antenna. These methods are of limited usefulness because, in the case of the X-ray method, the subject and the persons operating the device are exposed to potentially harmful X-rays. In the case of MRI tracing, the catheter cannot be used for imaging but only for catheter location. Therefore if an unexpected obstruction is encountered by the individual threading the catheter, additional interventional tools or imaging techniques must be used. This can result in increased possibility of injury for a patient, and increased difficulty of the procedure.
U.S. Pat. No. 5,348,010 to Schnall et al. discloses an inflatable MRI receiver coil employing a balloon. The tuning matching components in the Schnall device are placed outside the patient, thereby reducing the SNR of the received signal. Further, the balloon must be inflated during image acquisition, thereby occluding the entire diameter of the vessel in which it is placed, limiting or precluding its use in vascular applications where blood flow is desired during image acquisition, or, for extended periods of time, the airways. The distance between the receiver coil conductors in the Schnall device is also not fixed at any point along its inflation, which limits the tuning matching and decoupling components as they cannot be predetermined for a loop of a particular size while imaging.
There remains a need in the art for an MRI imaging device sleeve incorporating a flexible elongated MRI antenna suitable for a wide variety of interventional applications.
In accordance with the embodiments of the invention, systems and methods are provided herein for imaging using magnetic resonance imaging.
As used herein, the following terms generally encompass the following meanings, although these definitions do not limit the meaning of these words as would be understood by one of skill in the art.
xe2x80x9cInternally imagingxe2x80x9d generally denotes the acquisition of data interpretable as an image from an antenna situated within the confines of a structure to be imaged or within a body containing the structure to be imaged.
xe2x80x9cAdjacentxe2x80x9d generally denotes the condition of being inside of, next to, or in proximity of an object of reference. It may also denote the condition of being within the same body that contains the object of reference.
xe2x80x9cDetector coil,xe2x80x9d xe2x80x9cimaging coil,xe2x80x9d and xe2x80x9ccoilxe2x80x9d are synonymous terms that generally denote any arrangement of an electrically conductive and magnetic resonance compatible material acting as an antenna to receive and convey magnetic resonance data.
xe2x80x9cSleevexe2x80x9d generally denotes an object which surrounds a lumen or may be considered hollow by one of ordinary skill in the art. It may be of any shape. However, a sleeve will often refer to a tubular shape herein.
xe2x80x9cImaging sleevexe2x80x9d generally denotes a sleeve attached to a detector coil for internally imaging.
xe2x80x9cMRI sleevexe2x80x9d generally denotes an imaging sleeve dimensionally and/or constitutionally adapted for use in magnetic resonance imaging.
xe2x80x9cDimensionally differentxe2x80x9d generally denotes the condition in which one state of an object of reference differs from another state by the shape of the volume of space occupied by the object.
xe2x80x9cProbexe2x80x9d generally denotes any object that is adapted for passage through a substantially tubular member.
Certain embodiments comprise an apparatus for internally imaging using magnetic resonance imaging, having a first substantially tubular member including a distal and a proximal end and an interior and exterior surface, and a detector coil attached to the tubular member for internally imaging using MRI. In an embodiment, the detector coil is attached in proximity to the distal end of the tubular member. In another embodiment, the detector coil is located on the exterior surface.
In yet another embodiment, the detector coil is embedded within the tubular member. In another embodiment, the apparatus further comprises an electrical transmission member for electrically connecting the detector coil to an MRI scanner. In an embodiment, the electrical transmission member is located on the exterior surface of the first tubular member. In an embodiment, the electrical transmission member is a coaxial cable. In an embodiment, the electrical transmission member is a triaxial cable.
In one embodiment, the apparatus further comprises a second substantially tubular member placed coaxially with the first substantially tubular member. In an embodiment, the second tubular member is slideably related to the first tubular member.
In an embodiment, the detector coil includes at least one of a loop coil, a quadrature loop coil, a loopless coil, a loop expandable coil, a quadrature loop expandable coil, or a loopless expandable coil. In an embodiment, the first tubular member is dimensionally adapted for insertion into a body. In an embodiment, the first tubular member is dimensionally adapted for passage of medical devices therein.
In an embodiment, the detector coil resides on a flexible circuit board. In an embodiment, the detector coil comprises a solenoid.
In an embodiment, the apparatus further comprises a probe. In an embodiment, the probe includes a probe detector coil. In an embodiment, the probe detector coil includes at least one of a loop coil, a quadrature loop coil, a loopless coil, a loop expandable coil, a quadrature loop expandable coil, or a loopless expandable coil.
In an embodiment, the apparatus further comprises an attachment point disposed at the distal end of the first tubular member to affix the tubular member to an attached device. In an embodiment, the attached device includes a medical device. In an embodiment, the attached device is permanently affixed to the first tubular member. In an embodiment, the attached device is temporarily attached to the first tubular member. In an embodiment, the apparatus may further comprise a connector hub disposed at the proximal end of the first tubular member. In an embodiment, the connector hub includes strain relief.
In an embodiment, the apparatus further comprises an interface system having a tuning/matching circuit and a decoupling circuit, and is interposed between the detector coil and an MRI imaging system.
In an embodiment, the exterior surface and interior surface are coated with a lubricious material. In an embodiment, the lubricious material includes at least one of polyvinylpyrrolidone, polyacrylic acid, or silicone.
An embodiment comprises an apparatus for imaging using magnetic resonance imaging (MRI) including a substantially tubular member having a distal end, a proximal end, and a lumen extending between said distal and said proximal end, and a detector coil for imaging, using magnetic resonance imaging (MRI), wherein the tubular member is moveable between at least two states relative to the detector coil, such that in the first state the detector coil is positioned within the lumen and in the second state the detector coil is extended beyond the lumen to permit imaging.
In an embodiment, the detector coil includes at least one of a loop expandable coil, a quadrature loop expandable coil, or a loopless expandable coil. In an embodiment, the detector coil in the second state is expanded. In an embodiment, the detector coil in the first state is dimensionally different from the detector coil in the second state. In an embodiment, the detector coil is placed in a subject in the first state and detects magnetic resonance in the subject in the second state. In an embodiment, the detector coil is dimensionally adapted for insertion into and advancement through a catheter. In an embodiment, the detector coil can image in the first state.
In certain embodiments, the apparatus may further comprise a body lumen obstruction device. In an embodiment, the apparatus may further comprise an interface system having a tuning/matching circuit and a decoupling circuit, and the interface system is interposed between the detector coil and an MRI imaging system.
Another embodiment provides a method for imaging using magnetic resonance imaging comprising placing a first and a second detector coil internal to a subject and adjacent to an area for imaging, generating magnetic resonance in the area, and moving the first detector coil relative to the second detector coil so that the coils in combination detect the magnetic resonance.
In an embodiment, wherein the step of placing, at least one of the first detector coil and the second detector coil can detect the magnetic resonance. In an embodiment, wherein the step of placing, magnetic resonance is generated.
Another embodiment provides a system for imaging using magnetic resonance imaging, comprising a first detector coil for internally detecting magnetic resonance, a second detector coil for internally detecting magnetic resonance, and a controller for using the first detector coil in combination with the second detector coil for detecting magnetic resonance in an area to be imaged.
Another embodiment provides a system for imaging using magnetic resonance imaging, comprising means for placing a first and a second detector coil internal to a subject and adjacent to an area for imaging, and means for moving the first detector coil relative to the second detector coil so that the coils in combination detect magnetic resonance.
Another embodiment provides an apparatus for internally imaging using MRI, comprising a detector coil for internally imaging using MRI, and a trigger mechanism in communication with the detector coil, wherein activation of the trigger mechanism causes the detector coil to change from a collapsed state to an expanded state. In an embodiment, the trigger mechanism comprises a pull wire. In an embodiment, the detector coil in the collapsed state is dimensionally different from the detector coil in the expanded state.